In fiber optics, optical links (optical fibers) require calibration to maximize the Optical Signal-to-Noise Ratio (SNR) on the link. Link calibration generally includes determining launch power of transmitters, determining optical amplifier (e.g., Erbium Doped Fiber Amplifier (EDFA)) gain settings, and determining actuator (e.g., Variable Optical Attenuators (VOAs), Wavelength Selective Switch (WSS) pixel settings, etc.) settings. The optimal launch powers are determined to maximize OSNR and minimize non-linear penalties for different fiber types. The optical amplifier gain settings can include determining and setting EDFA gain stages, also known as gain switch modes (selecting two-stage versus three-stage gain blocks per EDFA) to minimize Noise Figure (NF) based on the required gain to compensate for span loss. The actuator settings are determined in every span to meet the launch power target with minimal noise impact that includes setting for Raman gain, EDFA gains, Post-VOA attenuations, mid-stage equalization (if any) to minimize amplifier ripple and Stimulated Raman Scattering (SRS) impacts per span, etc. Specifically, link calibration includes determining the aforementioned settings for initially turning up an optical link. Once turned up, various photonic control loops can be used to determine the settings over time and based on feedback.
The conventional approach for link calibration includes modeling a system with the physical characteristics determined via a simulation or planning tool (e.g., offline). This considers the worst case insertion losses or in some cases using statistically distributed values based on a certain batch of manufactured data points. This further considers End of Life (EOL) conditions with full-fill spectrum, EOL repair margins, and component aging penalties. This further considers user inputs on per span fiber properties such as fiber type, fiber length, average fiber loss (in dB per km), an effective core area in fiber, etc.
Usually, simulations provide the expected parameter setting required for initial system turn up (i.e., link calibration) that includes defining target launch power and amplifier gain tilt for handling Wavelength Dependent Loss (EDL), setting gain switch modes on EDFAs, and setting recommended gain for Raman amplifiers. However, simulations typically are wrong in two places, namely wrong entries input into the simulation and wrong provisioning on the system. Wrong entries in the simulation can occur due to incorrect data, poor estimates, input error, etc. This can include system parameters for fiber type, length, average loss per distance, estimated patch panel losses, the location of splices in the fiber span where different fiber types are spliced together, etc. Of course, wrong entries generate errors in the calibration settings as well as on the expected system performance. Also, the wrong provisioning on the system can occur when simulation outcomes are manually set by the user in the network elements. By automating the provisioning steps from simulations to the network elements directly, this error step is eliminated. However, it is not possible to remove the errors due to the wrong entries as this data is not exactly known a priori (otherwise it would not be a simulation).
After the initial link calibration, closes loop controllers are run using measured system parameters to optimize performance, although the overall outcome remains dominated by the user provisioned initial targets as outlined above. That is if the initial settings are wrong or not aligned with the actual system measured parameters, the system will not be able to generate the best possible optical outcome. A good example is target launch power settings for Raman gain settings. System controllers will only operate to maximize performance to match the provisioned target. If that target is wrong, the overall performance outcome remains underachieved.
The goal is to support system calibration at start-up without having to rely on simulations. Further, setting up system parameters for C+L links will be further complicated, compared to only C-band or only L-band transmission, where each C+L links will have dedicated components to support C- and L-band transmission in the same fiber and will require explicit parameter settings for both bands for optimal operation.